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Aviation History
1930
UNTITLED0 - 0612.PDF
SUPPLEMENT TO FLIGHT MAY 30, 1930 THE AIRCRAFT ENGINEER ends. A single tube compression strut may be ideal from the point of view of making and attachment, but it demands that the lines of action of struts and wires should pass approxi- mately through the spar centre when looking along the spar. However, a single tube is frequently used with a strong rib placed near the fitting to take the torsional loads. Wires All wires used in general practice terminate in a screwed portion which is secured to either a fork-end or trunnion. The provision for attaching the fork-end to the joint may be a plate lug or a shackle of suitable size spanning the spar or drag strut attachment. The form of vibration known as " wire flutter " should be remembered when a plate lug is used to secure a fork-end. Modern streamline wires are weaker in a direction at right angles to the path of flight and thus tend to vibrate in this direction, frequently to an extent which may cause the wire to break if it is unduly long for its size. If the plate lug is bent or is flat in the direction of vibration there is a great tendency for the lug to fail in use due to fatigue of the metal taking place. Diagrammatic Description of a Strut Joint One of the most important points to elucidate in the design of a strut joint is the path taken by the loads in the members in order to ensure that each member is firmly anchored without causing stresses which have not been allowed for. The following essentials are necessary in order to commence work on the design of a strut joint—the type of spar to be used, the directions and angles of the various members and the loads in all members under the main conditions of flight. These conditions of flight are :— 1. Centre of pressure forward (C.P.F.) 2. Ditto, with front flying wire cut. 3. Centre of pressure aft (C.P.A.) 4. Ditto, with rear flying wire cut. 5. Nose Dive. In certain cases the landing and inverted flight loads may be necessary for particular members. It is advisable to have at hand the loads in all members for these conditions of flight, as under any one condition, the load in a member may add to others or subtract from them, and thus to arrive at a safe though economic design it is necessary to know the maximum resultant load in any par- ticular direction. To Wing Be - FLYING AND I IANUHG mKES-^* END VIEW LINtS Of ACTION Of MEMBERS MEETING ftT A STRiT JOINT ' DRAG STRUT. Dtttc mm LANDING mat iHT&PiANC. smsr i. INCIDENCE WIRE ' ANT) DRAG W1K In the diagram is shown a front spar and the lines of action of wires and struts, assuming the members have no offsets in any direction, and that all their lines of action pass through the centre lines of the spar. This is an ideal arrangement and is a point which might be aimed at when considering the general design of an aeroplane, due to the simple type of fitting which may then be evolved. Consider this hypothetical fitting under any single con- dition of loading, for instance centre of pressure forward (C.P.F.). Under this condition the landing wire, incidence wire and one or other of the drag wires will have no load in them, being redundant in the aeroplane structure, and may thus be left out of consideration when stressing for this par- ticular loading. Coming now to the effects the members and their loads may have upon the attachment to the spar, they may be split up into three important types by taking the component or resolved loads as follows :— 1. Along the spar, parallel to its centre line. ' • 2. Vertically, perpendicular to the spar centre line. 3. Horizontally or fore-and-aft perpendicular to the spar centre line. These will now be taken in turn, and it will be assumed that the main attachments to the spar are on the front and rear sides. (This is the most common form of joint, and enables a neat fitting to be evolved.) 1. Examining the diagram it can be seen that two members only can have their loads resolved along the spar centre line, these being the flying wire and one drag wire (one drag wire and the landing wire being inoperative). The flying wire load can be split up into two halves running along each side of the spar, while the drag wire load is taken along the rear side only, the wire being attached direct to the rear side. Thus we see that the loads exerted by the fitting on the spar are different on the front and rear sides, and whereas on the front side the attachment must be strong enough to take half the component flying wire load, that on the rear side must be strong enough to take the same load plus the anti-drag wire component load, or it may be relieved to the extent of this load, depending upon which of these two latter wires is in operation under the C.P.F. condition. These loads along the spar are the most important in the joint in that they usually have the greatest magnitude, and it is essential that they should be well cared for. 2, The loads which may be taken vertically are the flying load in the spar, the load in the interplane strut, which are both vertical, and the vertical component of the flying wire acting in a direction opposite to that of the other two. In actual practice it will usually be found that the plates which form the flying wire attachment are a part of, or are directly connected to the strut attachment, and thus the upward load of the strut iB taken by the flying wire without actually going into the spar. We are left now with the flying or lift load in the spar which tends to lift it away from the fitting, and this must be taken into consideration as it may cause an undue crushing effect on the spar, or an unlooked-for bending effect on the attachment bolts. In certain cases the flying wire and strut attachments may be independent, and then each must be effectively secured, while care must be taken that there is a rigid path for the strut load to get on to the flying wire attachment. The strength of the flying wire fixing is generally sufficient for the spar flying load. 3. In the fore-and-aft direction we have the components of the drag or anti-drag wire and the drag strut, and the loads in these members approximately balance. Thus the strength of the attachment required under case 1 is amply sufficient for this case. There is one important point which should be remembered with respect to these members, and this depends upon the form of drag strut used. If the latter is of the single-tube type lying in the same plane as the drag wire centre line, the consideration does not arise. But if the drag strut is of the built-up type, demanding some form of channel or vertical angle for its attachment, then as the drag wire will be taken off some particular point of this channel, say, in the middle, the channel will be in bending due to the opposing forces in the two members. This may be rather a vital point as if the built-up structure is deep and the channel long, and is unsupported, or only partially sup- ported, by the spar, the bending effect will reach a high value and should be seriously considered in the general design. The above considerations complete the investigation for the C.P.F. condition which is always the most important for the front spar fittings, as C.P.A. condition is the most important for rear spar fittings. We have as yet taken no loads for the incidence wire or landing wire, because these members come into use under other conditions of-loading and they will now be examined separately. 5860
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